In recent years, a new type image forming apparatus, which is, for example, shown in Publication Gazette of Unexamined Japanese Patent Application Hei 6-95489, has been proposed. In the conventional image forming apparatus, a cylindrical developing electrode having a magnet coaxially provided with its rotation axis is disposed parallel to a cylindrical photoconductor with a predetermined developing gap. A gap restricting member is provided between the photoconductor and the developing electrode. The developing electrode is pressed on the gap restricting member, for example, by a spring, and the gap restricting member contacts the photoconductor. Thus, a width of the developing gap is defined by a thickness of the gap restricting member.
There are decentering and deflection in the photoconductor and developing electrode. Since the developing electrode is pivoted by an arm, effects of the decentering and deflection are cancelled by revolution movement of the developing electrode around a rotation axis of the arm. The developing electrode and magnet are not moved reciprocally but revolve against an axial standard line which links the rotation axes of the photoconductor and developing electrode. Thus, an angle of a magnetic center line linking a position of the magnetic pole and the rotation axis of the developing electrode against the axial standard line will be changed. When the magnetic pole is displaced from the original setting position, gradation of a picture image will be damaged or background development of toner will occur in non-image part of a paper sheet responding to rotation frequency of the photoconductor or developing electrode.
A condition that a setting angle of the magnet is changed by revolution of the developing electrode is schematically shown in FIG. 19. In FIG. 19, the original positions of the rotation axis of the photoconductor O, the rotation axis of the developing electrode Q, the axial standard line L, the magnetic center line QM and the rotation axis P of the arm are shown by real lines. Under this condition, the distance between the rotation axis O of the photoconductor and the rotation axis Q of the developing electrode changes due to the decentering and deflection of the photoconductor and the developing electrode. At this time, the developing electrode moves to a position shown by dotted line by the revolution around the rotation axis P of the arm. As a result, the rotation center Q of the developing electrode moves to a position designated by Q', and the axial standard line L changes to a line designated by L'. Furthermore, the magnetic center line QM changes to a line designated by Q'M'.
A crossing angle .THETA. of the lines L and M changes to a crossing angle .THETA.' of lines L' and M'. In these angles, a relation of .THETA.'-.THETA.=.delta.1+.delta.2 is concluded. Namely, the setting angle of the magnetic pole against the axial standard line L is changed by (.delta.1+.delta.2). This phenomenon is caused by not the reciprocal movement but the revolution movement of the developing electrode and the magnet against the axial standard line L.
Especially, an image forming method shown in Publication Gazette of Unexamined Japanese Patent Application Hei 6-95489, magnets are provided not only in the developing electrode but also in the photoconductor. Therefore, not only a balance but also the relative positions of the magnets become more important. Minute changes of the setting angles of the magnets largely change the magnetic field generated in a developing nip part, and largely affect the quality of picture images.
Furthermore, the developing electrode and the photoconductor are respectively borne by different housings of developing unit and photoconductor unit. Thus, accuracy of the position of the photoconductor against the developing electrode cannot be guaranteed. As a result, quality of the picture image is largely reduced.